1. Field of the Invention
The present invention relates to a quartz glass jig, such as a quartz glass reaction tube (hereinafter referred to as a reaction tube) and a quartz glass wafer boat (hereinafter referred to as a wafer boat) that are used in a heat-treatment apparatus for semiconductor wafers, such as a CVD apparatus or a thermal diffusion apparatus.
More particularly, the present invention relates to a quartz glass jig, such as a reaction tube or a wafer boat, used in a thermal CVD apparatus.
2. Related Prior Art
Conventionally a CVD apparatus was well known in which a chemical reaction in a activated space was used to form a thin film on a wafer or wafers.
There are, for example, a thermal CVD apparatus, a plasma CVD apparatus and a photo-assisted CVD apparatus which respectively use heat, plasma and ultraviolet light or laser as activation energies.
In a thermal CVD apparatus, vapor phase chemical reaction (for example, thermal decomposition, hydrogen reduction, oxidation or substitution reaction) at a high temperature of a volatile metal hologenide, a volatile metal organic compound or the like is conducted to form an epitaxial film made of semiconductor such as silicon or a metal having a high melting point, or an insulating film or a protection film made of the oxide of semiconductor such as silicon on a wafer or wafers.
Jigs such as a reaction tube and a wafer boat used in such CVD apparatuses are generally made of quartz glass from the requirements for chemical stability and heat resistance.
In the vapor phase growth, since the surface or surfaces of a quartz glass jig are exposed to a vapor phase growth space in the apparatus, a thin film or films adhere on the surface or surfaces of the jig as well as the surface of a wafer or wafers.
A reaction temperature of the vapor phase chemical reaction required for thermal decomposition of SiH4, which is used for formation of a Si thin film in a thermal CVD method, is in the range of 500xc2x0 C. to 1100xc2x0 C.
A reaction temperature of the vapor phase chemical reaction for reduction of SiCl4 is upto 1200xc2x0 C.
What""s more, because the thermal expansion coefficients of a Si film and quartz glass, which is the material of a reaction tube, are different from each other by one figure or so, the film has a risk to get separate due to a stress caused by the difference between the expansion coefficients, when a CVD film is thin.
On the other hand, when the CVD film is thick and the stress affecting the quartz glass of the reaction tube is larger, the reaction tube, this time, has a risk to break.
This is also the case with not only a CVD apparatus but also a diffusion furnace in which the heat-treatment space is under influence of a high temperature of about 1200xc2x0 C., and especially the above-mentioned adverse effects is conspicuously observed in a CVD apparatus in which thin films adhere on surfaces.
In the case of a wafer boat in the reaction tube aforementioned where wafers are disposed in a stacking manner at respective predetermined positions, boat marks (which means poor exterior appearance observed on the surface or surfaces of a wafer caused by contacting with the boat and sometimes are accompanied by chipping along the periphery of a wafer) often occur at a contacting portion or portions between the periphery of each of the wafers and the boat, because the peripheries of the wafers and the boat are in the contacting conditions of planes or lines and move over each other with friction due to the difference between the thermal expansion coefficients.
The present invention was made to solve the technical faults of the prior art.
It is an object of the present invention to provide a quartz glass jig used for a heat-treatment apparatus for semiconductor wafers, in particular a CVD apparatus, and a method for producing the jig that can solve various faults caused by the difference between the thermal expansion coefficients of the aforementioned thin film and the quartz glass jig.
It is another object of the present invention to provide a reaction tube to be used in a CVD apparatus in which neither crack nor breakdown occurs, even when a thin film adheres on the internal wall or walls of the reaction tube.
It is a further object of the present invention to provide a wafer boat to be used in a CVD apparatus in which neither boat mark on nor chippings along the periphery of a wafer or wafers occur.
The present invention is applied to a quartz glass jig to be used in a heat-treatment apparatus, in particular a CVD apparatus, and is characterized in that the jig is made of transparent quartz glass as the material and that sand-blasting is given on the portion or portions exposed to the heat-treatment space.
The reason why the material of the jig is limited to transparent quartz glass is that transparent quartz glass has a more uniform surface hardness in comparison with bubble-mixed or opaque quartz glass and a desired surface roughness is effectively achieved in sand-blasting, so that the faults resulted from the stress occurred between the CVD film and a treated surface or surfaces of the jig are effectively prevented.
In the present invention, in a more concrete manner, in the case of a reaction tube 1 used in a thermal CVD apparatus in FIG. 1, at least the portion or portions which are heated by a heater 2 of the internal wall 10a are preferably sand-blasted and in the case of a wafer boat 20 used in a thermal CVD apparatus in FIG. 2, at least the portions of the wafer boat 20 that contact with each wafer 30 are preferably sand-blasted.
According to the present invention, the center-line mean roughness (Ra) of the surface after sand-blasting is set in the range of 1 xcexcm to 20 xcexcm, preferably 2 xcexcm to 10 xcexcm, more preferably 3 xcexcm to 5 xcexcm. It is still more preferable that the maximum height (Rmax) is roughly set in the range of 10 xcexcm to 30 xcexcm, while the center-line mean roughness (Ra) is set in the range of 3 xcexcm to 5 xcexcm.
Abrasive material used in sand-blasting is made of a material or a mixture of materials that are harder than quartz glass and do not become a contamination source or sources, for example, crystallized quartz powders or SiC powders and the grit size distribution is preferably set so that the weight in the range of 100 xcexcm to 400 xcexcm in grit size is at 90% of the total weight.
Such a function of the present invention will be explained in reference to FIG. 3(a) and FIG. 3(b) below:
For example, a case that a poly silicon film 40 adheres on the internal wall surface 10a of a reaction tube 10 in a CVD method is here considered referring to FIG. 3(a).
Since the linear expansion coefficient of Si is 4.7xc3x9710xe2x88x926/xc2x0 C. to 4.9xc3x9710xe2x88x926/xc2x0 C. and that of SiO2 is 5.5xc3x9710xe2x88x927/xc2x0 C., thermal contraction of the poly silicon thin film 40 occurs, but that of quartz almost no way occur, when the temperature is lowered from any in the range of 550xc2x0 C. to 770xc2x0 C. to room temperature of 20xc2x0 C. during CVD growth.
In this circumstance, the poly silicon thin film 40 makes a compressive stress A occur in the quartz glass 10 and on the other hand the quartz glass 10 makes a tensile stress B occur in the poly silicon thin film 40.
The thermal stress A, which the poly silicon thin film 40 gives to the quartz glass 10, accordingly is obtained from the following equation:
A=E(xcex11xe2x88x92xcex12)xcex94T=(1.9xcx9c2.4)xc3x97108dyne/cm2,
where E: Young""s modulus of poly silicon thin film 40 (0.9xc3x971011dyne/cm2)
xcex11: linear expansion coefficient of poly silicon thin film 40 (4.5xe2x88x9210xe2x88x926/xc2x0 C.)
xcex12: linear expansion coefficient of quartz (5.5xc3x9710xe2x88x927/xc2x0 C.)
xcex94T: (550xcx9c700)xe2x88x9220=530xcx9c680xc2x0 C.
If the thermal stress A is (1.9xcx9c2.4)xc3x97108 dyne/cm2, this value exceeds the design stress for a reaction tube and the breakdown thereof thus becomes a real risk, in the case that the dimensions of the reaction tube are set at respectively 350 mm in diameter and 4 mmxcx9c5 mm in the thickness of the wall.
In the mean time, when the surface of the quartz glass 10 is sand-blasted, the surface is roughened to show a level of irregularity as shown in FIG. 3(b).
If a CVD treatment is carried out in this state, a poly silicon thin film 40 is formed on the surface in conformity with the roughened surface profile.
Such a poly silicon thin film 40 forms a compression stress B in the surface of the quartz glass which has a serrate contour in correspondence to the irregularity of the original surface of the quartz 10.
This causes divergence of the stress in the quartz glass.
On the side of the quartz glass 10, the peaks in the surface irregularity of the quartz glass 10 does not have enough strength and, therefore, with the stress A occurring on the side of the quartz glass 10, only the peaks are broken down and the propagation of the stress into the bulk is stopped within the vicinity of the surface, so that the break-down and a crack or cracks of the quartz glass can be prevented.
In the case of a wafer boat installed in the aforementioned reaction tube with which wafers 30 are disposed in a stacking manner at respective predetermined positions, the wafers 30 have each a linear contact relation in the holding groove 21, which is formed in the wafer boat 20 together with a plurality of the other same grooves 21.
The difference therebetween in the thermal coefficients causes boat marks on or chippings at the peripheries of the wafers 30.
If a wafer boat is made of transparent quartz glass material which is excellent in flatness and smoothness, this faults are increased more, because of occurrence of a linear contact relation, which puts a wafer in a closer geometrical condition with the boat.
By sand-blasting the internal surfaces of a holding groove 21 mentioned above, which is formed in the wafer boat 20, since the wafer 30 is placed on the peaks of the surface irregularity as illustrated in FIG. 4(b), the wafer 30 and the wafer boat 20 are a point contact relation with each other at a point P, and the wafer 30 does not run a risk to have boat mark or chippings thereon due to divergence of the thermal stress acting in the surface of the wafer 30, which would otherwise acts straight.